Math troubles with addition

My script is conjoining the two values instead of adding them together.
So instead of 2 + 2 = 4, 2 + 2 = 22

calculate_btn.onPress = function() {
textbox3_txt.text = textbox1_txt.text + textbox2_txt.text
}

textbox1 = input text with instance name of textbox1_txt
textbox2 = dynamic text with instance name of textbox2_txt
textbox3 = dynamic text with instance name of textbox3_txt

The problem is only with addition. Multiplication, division and subraction all work fine.